When creating a poll you now have the option to allow people to change their votes.
New & Existing polls default to this disabled.
Polls that exist today can be edited to enable vote changing.
There are no limits to how often votes can be changed but I will probably get around to limiting it to one change (period, not per poll) a calendar day to limit the people who like to change their minds to an annoying degree.
When you've voted if changes are enabled you will have the option to "Change your vote". Clicking this effectively un-votes you and let's you pick a new (or even the same) option, or leaves you able to abstain by simply not voting again.